Urethane dimethacrylate (UDMA), an oligomeric poly(ethylene glycol) extended UDMA (PEG-U) and a blend of UDMA/PEG-U were chosen as model systems for introducing both hydrophobic and hydrophilic segments and a range of compliances in their derived polymers. Experimental composites based on these three resins with amorphous calcium phosphate (ACP) as the filler phase were polymerized and evaluated for mechanical strength and ion release profiles in different aqueous media. Strength of all composites decreased upon immersion in saline (pH = 7.4). Both polymer matrix composition and the pH of the liquid environment strongly affected the ion release kinetics. In saline, the UDMA/PEG-U composite showed a sustained release for at least 350 h. The initially high ion release of the PEG-U composites decreased after 72 h, seemingly due to the mineral re-deposition at the composite surface. Internal conversion from ACP to poorly crystallized apatite could be observed by X-ray diffraction. In various lactic acid (LA) environments (initial pH = 5.1) ion release kinetics was much more complex. In LA medium without thymol and/or carboxymethylcellulose, as a result of unfavorable changes in the internal calcium/phosphate ion stoichiometry, the ion release rate greatly increased but without observable conversion of ACP to apatite.

Recent research in our laboratories has focused on similar problems in bioactive resin-based composites that utilize a unique calcium phosphate, amorphous calcium phosphate, ACP, as the filler phase rather than the conventional silanized glass fillers (1). This type of composite is capable of providing a sustained release of calcium and phosphate ions in aqueous milieus such as exists in the oral cavity (2). Significantly, this type of composite has the potential for repairing and preserving contiguous mineralized tissues such as enamel and dentin (3). While there have been some promising improvements recently (3-6) with regard to mechanical properties, the less than optimal PS and SD, along with DC are still considered shortcomings of these as well as conventional glass-filled composites.This study was designed to determine the effects of a new high molecular mass oligomeric urethane dimethacrylate co-monomer (PEG-U) on PS, SD and DC. In the past, urethane dimethacrylate (UDMA), a commonly used dental monomer, has been blended with 2hydroxyethyl methacrylate (HEMA) in formulating ACP composites, see Fig. 1 for structures of UDMA, HEMA and PEG-U. Specifically, the aim of this study was to assess the effects of the UDMA resin matrix composition on DC, PS, SD and the biaxial flexure strength (BFS) of ACP composites when PEG-U was used in place of HEMA.Disclaimer: Certain commercial materials and equipment are identified in this article to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, Food and Drug Administration or American Dental Association Foundation or that the material or equipment identified is necessarily the best available for the purpose.